UNITED NATIONS — Careful not to blame either side for a deadly chemical weapon attack, U.N. inspectors reported Monday that rockets loaded with the nerve agent sarin were fired from an area where Syria’s military has bases, but said the evidence could have been manipulated in the rebel-controlled stricken neighborhoods.

The U.S., Britain and France jumped on evidence in the report — especially the type of rockets, the composition of the sarin agent, and trajectory of the missiles — to declare that President Bashar Assad’s government was responsible.

Russia, Syria’s closest ally, called the investigators’ findings “deeply disturbing,” but said it was too early to draw conclusions. The Syrian government’s claims that opposition forces were responsible for the attack “cannot be simply shrugged off,” Russian Ambassador Vitaly Churkin insisted.

The conclusions represented the first official confirmation by impartial scientific experts that chemical weapons were used in Syria’s civil war, but the inspectors’ limited mandate barred them from identifying who was responsible for the Aug. 21 attack.

“This is a war crime,” U.N. Secretary-General Ban Ki-moon told the Security Council when he presented the report. “The results are overwhelming and indisputable. The facts speak for themselves.”

Ban called it “the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them” in Halabja, Iran, in 1988, and “the worst use of weapons of mass destruction in the 21st century.”

The deep division between Western backers of rebels seeking to overthrow Assad and Russian and Chinese supporters of the regime has paralyzed the U.N. Security Council since the Syrian conflict began 2½ years ago.

After months of negotiations, the U.N. inspectors went to Syria to visit the sites of three alleged chemical attacks earlier this year and were in the capital of Damascus on Aug. 21 when reports and videos began surfacing of a shelling attack in which victims experienced shortness of breath, disorientation, blurred vision, nausea, vomiting, weakness and a loss of consciousness.

They finally gained access to three towns where the Aug. 21 attack occurred, and on one occasion their convoy was hit by sniper fire, but the inspectors were nonetheless able to collect a large amount of material and talk to survivors and witnesses.

The report cited the following evidence to support its conclusions:

Rockets and fragments were found to contain sarin. “Several surface-to-surface rockets capable of delivering significant chemical payloads were identified and recorded at the investigated sites,” the investigators said.

Close to the impact sites, in the area where people were affected, inspectors collected 30 soil and environmental samples — far more than any previous U.N. investigation — and in a majority of the samples, “the environment was found to be contaminated by sarin,” its by-products, and “other relevant chemicals, such as stabilizers.”

Blood, urine and hair samples from 34 patients who had signs of poisoning by a chemical compound provided “definitive evidence of exposure to sarin by almost all of the survivors assessed.”

More than 50 interviews with survivors and health care workers “provided ample corroboration of the medical and scientific results.”